Osteoblasts (Obs) differentiate from mesenchymal stem cells (MSCs) and polarize on bone surfaces to produce new bone that fills in cavities created by osteoclast-mediated bone resorption. Polycystins 1 and 2 (Pkd1/Pkd2) and TAZ form a mechanosening complex in mature Obs that plays an important role in the maintenance of bone mass. Genetic loss of Pkd1/TAZ in mature Obs of mice results in osteopenia caused by decreased Ob-mediated bone formation (Ob-BF) and an unexpected increase in bone marrow fat (MAT). This inverse relationship between Ob-BF and marrow adipocytes in compound Pkd1/TAZ deficient mice resembles certain forms of human osteoporosis (OP). Increased MAT in OP is thought to be due to a shift of MSC differentiation to adipocytes at the expense of Obs, and the release of paracrine factors by adipocytes are purported to inhibit Ob- BF. However, increased MAT resulting from disruption of Pkd1/TAZ in mature Ob is not readily explained by effects on lineage commitment. Rather, the direct effects of Pkd1/TAZ in Obs raises the possibility that other mechanisms are responsible for converting Obs to adipocytes. An unexplored possibility is that adipocytes arise from transdifferentiation of Obs. We propose the novel hypothesis that physical forces in the bone microenvironment regulate Ob transdifferentiation to adipocytes (OAT) through activation of Pkd1/Pkd2/TAZ. The R61 phase will: 1) explore the mechanisms whereby Pkd1/Pkd2/TAZ in Obs inversely regulate Ob-BF and MAT in bone, and 2) develop pharmacological tools to modify this process. In Aim 1, mouse genetic approaches will be used to conditionally delete Pkd1 and TAZ in osteoblasts and define the functional interdependence of Pkd1 and TAZ in regulating bone mass. In Aim 2, lineage tracing studies will determine the respective roles OAT or altered lineage commitment in the reciprocal regulation of Ob-BF and MAT in these mice. In Aim 3, we will pursue pre-therapeutic lead design and optimization of novel small molecules that ?staple? the C-terminal regions of Pkd1 and Pkd2, and stimulate Pkd1/Pkd2/TAZ to promote osteoblastogenesis and inhibit adipogenesis in Ob cultures. Establishing that Pkd1/Pkd2/TAZ regulates OAT in vivo would be a paradigm shift that challenges prevailing concepts of how bone mass is maintained in health and disease. Optimizing lead analogues that activate this complex would potentially provide a novel treatment for OP. In Aim 4, which will be undertaken in the R33 phase only if the expected outcomes of Aims 1-3 are achieved, we will synthesize sufficient quantities of the lead compounds to characterize in vivo pharmacokinetic (PK) properties and safety profiles, and test the ability of the most promising compounds to prevent bone loss in preclinical mouse models of OP. A role of the mechanosensing complex Pkd1/Pkd2/TAZ to regulate Ob to adipocyte conversion provides a new perspective for understanding the pathological significance of the reciprocal relationship between bone formation and MAT in OP and defines a new therapeutic target for developing drugs to treat OP that uniquely stimulate Ob-mediated bone formation and inhibit MAT through activation of Pkd1/Pkd2/TAZ in mature osteoblasts.